1. Technical Field
The present invention relates to an interfacial adhesion strength measuring apparatus and method of a gas diffusion layer for fuel cells, and more particularly, to an interfacial adhesion strength measuring apparatus and method of a gas diffusion layer for fuel cells that quantitatively detect an applied load when fracture or peeling is generated in the gas diffusion layer by adhering the gas diffusion layer to a pair of cantilevers using different adhesives and then applying force to the gas diffusion layer in a direction in which the gas diffusion layer is separated.
2. Description of the Related Art
A stack for a fuel cell vehicle is generally configured of several hundreds of unit cells, each of which includes a membrane electrode assembly (MEA), a gas diffusion layer (GDL), and a separator. Among these components of the unit cell, the gas diffusion layer serves to uniformly supply hydrogen, air, or oxygen, which is reaction gas supplied along channels of a separator, to a surface of the membrane electrode assembly.
In particular, the gas diffusion layer includes a carbon fiber substrate 110 that operates as a support and has a paper form, a cloth form, or a felt form, and a micro porous layer (MPL) 130 present on the substrate 110 in a form in which it is coated, as shown in FIG. 1, wherein the substrate 110 and the micro porous layer 130 have an interface present therebetween. However, in many gas diffusion layers, an interphase region is formed in a shape in which carbon pastes having a predetermined viscosity partially penetrate into a surface of the porous substrate 110 when the micro porous layer is coated, rather than forming a clear interface. A thickness of the interphase region varies depending on a structure of the substrate 110 and a feature of a material of the micro porous layer 130. In addition, the interfaces are structurally more fragile particularly under harsh fuel cell operating conditions. For example, when the stack repeatedly experiences a dry/wet or freeze/thaw cycle environment, a problem such as fracture, peeling, or the like, is first generated on the interface, which has a substantial influence on durability of the fuel cell stack. In other words, interfacial adhesion has been recognized as an important factor capable of determining a lifespan of a fuel cell vehicle.
Generally, as a method of measuring the interfacial adhesion, a scotch tape test method, a scratch test method, and the like, which are indirect and qualitative methods, and a pull-off test method, a peel test method, and the like, which are direct and quantitative methods have been developed. The scotch tape test method is a method of evaluating the adhesion by attaching an adhesive tape having a predetermined length to the surface of the micro porous layer 130 and then detaching the adhesive tape from the surface of the micro porous layer 130. When the micro porous layer 130 is attached to the adhesive tape to be detached from the substrate 110, adhesion between the adhesive tape and the micro porous layer 130 is greater than adhesion between the micro porous layer 130 and the substrate 110. However, this method may determine only whether an examination is passed.
Further, the scratch test method is a method of estimating adhesion by a threshold load value when the micro porous layer 130 is peeled by moving the substrate 110 while increasing a load on the surface of the micro porous layer 130 using a stylus having a round end portion. This method has an advantage that it may be simpler to prepare a sample compared to other methods, such that rapid measurement may be possible, but has a disadvantage that a relationship between a threshold load and actual adhesion of the micro porous layer 130 may not be clearly discovered. The peel test method, which was developed from the scotch tape test method and is devised to quantitatively measure adhesion, is a method of calculating peeling strength by measuring an applied load while detaching a flexible strip having a predetermined width from the firm substrate 110 at a predetermined speed and dividing the measured load with the width of the strip. However, the peel test method may not be applied to a gas diffusion layer in which both of portions that correspond to the strip and the substrate are made of a soft material.
In addition, there is a method of creating a stable crack in the interface between the micro porous layer 130 and the substrate 110 through indentation and then measuring a threshold load required for advancing the crack to calculate adhesion of the micro porous layer 130 based on the fact that the advance of the crack is associated with interface fracture toughness and interfacial adhesion strength. However, in the case of the gas diffusion layer, the crack is not advanced along the interface at the time of the indentation, and both of the substrate 110 and the micro porous layer 130 are damaged. Therefore, this method may not be applied to the gas diffusion layer.
As described above, the existing interfacial adhesion evaluating methods and apparatuses are useful for measuring interfacial adhesion of a thin film coated on a firm and rigid substrate such as a metal, but may not quantitatively evaluate interfacial adhesion of a thin and flexible product such as the gas diffusion layer. In the peel test method or the scratch test method using an adhesive tape it may be difficult to compare qualitative and approximate relative adhesion, and requires improvement in several aspects such as efficiency, a cost, a time, and the like. Therefore, the development of an apparatus capable of quantitatively evaluating interfacial adhesion of a gas diffusion layer, which is one of electricity generating components for a stack, has been required.
The matters described as the related art have been provided only for assisting in the understanding for the background of the present invention and should not be considered as corresponding to the related art known to those skilled in the art.